A market of mobile communication including portable phones is expanding, and high performance of its service is under progress. As frequency band used by mobile communication gradually shifts to a frequency band higher than giga hertz (GHz), there is a tendency for mobile communication to become multi-channel. A possibility of future introduction of software radio (SDR: software-defined-radio) is being studied vigorously. In order to realize software radio, a wider adjustment range of circuit characteristics is desired.
FIG. 4 is a circuit diagram of a conventional frequency variable filter 100j. The variable frequency filter 100j has a plurality of channel filters 101a, 101b, 101c . . . , and switches 102a and 102b. By switching the switches 102a and 102b, any one of the channel filters 101a, 101b, and 101c . . . is selected to change the frequency band. A high frequency signal input from an input terminal 103 is filtered by the selected channel filter 101 and is output from an output terminal 104.
The frequency variable filter 100j has channel filters corresponding in number to the number of channels. A multi-channel increases the number of channel filters, complicates the structure, and increases the size and cost. A possibility of realizing SDR is small.
Attention has been paid recent years to a compact micro machine device using MEMS (micro electro mechanical systems). An MEMS device (micro machine device) using MEMS is able to have a high Q (quality factor) and be applied to a high frequency band variable filter (Patent Documents 1 and 2, Non-Patent Documents 1, 2, and 3). Since an MEMS device is compact and has a low loss, it is often used for a CPW (coplanar waveguide) distributed constant resonator.
Non-Patent Document 3 discloses a filter having the structure that a plurality of variable capacitors of MEMS devices ride over a three-stage distributed constant line. In this filter, a control voltage Vb is applied to a drive electrode of a MEMS device to displace a variable capacitor, change a gap to a distributed constant line, and change an electrostatic capacitance. Change in the electrostatic capacitance changes the pass band of the filter.
[Patent Document 1] JP-A-2008-278147
[Patent Document 2] JP-A-2010-220139
[Non-Patent Document 1] D. Peroulis et al, “Tunable Lumped Components with Applications Reconfigurable MEMS Filters”, 2001 IEEE MTT-S Digest, p 341-344
[Non-Patent Document 2] E. Fournet et al, “MEMS Switchable Interdigital Coplanar Filter”, IEEE Trans. Microwave Theory Tech., vol. 51, No. 1 p 320-324, January 2003
[Non-Patent Document 3] A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capatitors”, IEEE Trans. Microwave Theory Tech., vol. 51, No. 7, p 1878-1885, July 2003